The melanopsin expressing intrinsically photosensitive retinal ganglion cells (mRGCs or ipRGCs) are necessary for adapting non-image forming behavior and physiology of the animal to the ambient light conditions. Melanopsin photopigment uses a signaling mechanism that is distinct from that of the classical rod/cone opsins. Very little is known about the molecular events following melanopsin activation. Specifically, molecules and mechanisms mediating photopigment desensitization, internalization, degradation or regeneration of functional photopigment are unknown. These steps in receptor function determine the threshold sensitivity, adaptation and temporal integration parameters. We have determined the C-terminus cytoplasmic region of melanopsin is phosphorylated at multiple sites and is required for its functional interaction with arrestin and subsequent desensitization. Experiments proposed in this application will evaluate the relevance of melanopsin phosphorylation and arrestin interaction in melanopsin mediated photoresponses in vivo. In cultured cells we will test whether light activated melanopsin undergoes endocytosis and degradation and assess the role of melanopsin phosphorylation, and arrestin interaction in this process. Subsequently, we will develop strategies to specifically perturb gene expression in the mRGCs of adult mice. This approach will help evaluate the minimum threshold of melanopsin protein and melanopsin expressing ganglion cells required for normal non-image forming photoresponses. Using this ipRGC specific gene expression strategy we will perturb melanopsin phosphorylation by altering kinase activity or by expressing melanopsin with mutated phosphorylation sites. Next, we will assess melanopsin function in mice lacking individual beta-arrestins. Mice with perturbations in melanopsin phosphorylation, kinase activity or arrestin expression will be subject to various tests to evaluate light dependent melanopsin degradation, response kinetics of ipRGCs, and behavioral adaptation to light. Results from these experiments will illustrate the role of activity dependent phosphorylation of melanopsin in normal adaptation of behavior and physiology to light.